According to the World Food Organization, nearly half of all root and tuber crops worldwide are not consumed, but are lost due to inappropriate storage and post-harvest losses. In developing countries such as Ethiopia, potatoes have not been dried, but are traditionally stored in potato clamps. So far, dried potatoes have not been converted into usable foods.
The aim of the present work is to convert potatoes - perishable rootlets and tubers - into stable products by hot air drying. Hot air dryers are economical to operate in industrialized countries. In Africa, this is reserved for larger industrial companies only. In regions with a tropical climate, however, the use of solar tunnel dryers is worthwhile. These are a good choice for farming and small industries and wherever electrical energy is difficult or impossible to obtain.
In a first part of the work, the drying process of potatoes was investigated, in particular with regard to the change of thermal, mechanical and chemical quality parameters. In an evaluation of the literature it was found that potatoes are not subject to quality changes if the water activityis below a value of 0.2. In order to determine the water content associated with this value at storage temperature, the known equations for the sorption equilibrium were evaluated and verified with own experimental investigations. This determined the end point of the drying process.
The following experimental investigations showed a process-dependent change of the quality criteria such as color, shrinkage, and mechanical properties as well as the content of valuedetermining substances such as vitamin C and starch. The differences in the course and magnitude of the quality changes were attributed to the glass transition that takes place during the drying process. For the determination of the glass transition temperature a new, simple method based on the measurement of mechanical properties could be developed. The knowledge of the glass transition temperature allowed optimizing the drying process. The drying process could be carried out in the rubbery or glassy region, depending on the expected quality changes. Thus, all information was available to produce high quality dried potatoes in an industrial process.
Since the production of potato products in less industrialized regions without sufficient supply of electrical energy should be included, potatoes were dried with a solar tunnel dryer. Examination of the quality properties mentioned above confirmed the process-dependent quality changes.
Finally, the dried product was ground and with the flour thus produced, wheat flour was replaced for baking bread. An evaluation of the finished bread by a panel showed that the acceptance of the bread according to the new recipe was high, also with regard to baking volume, taste, texture and color.
This work shows that by drying potatoes can be transformed a well accepted, storable and easily transportable product. The risk of losses or degradation is minimized. It can be produced on an industrial as well as on farm level. If the influence of the glass transition is taken into account, it is possible to optimize the quality of the product.